I don't think you can have read my post: it was an Easyjet positioning flight that flew over my house. That doesn't sound like an air test.
positioning from where? in which area do you live?
the aircraft will drop out of any stack when it can and make its way towards the approach at whatever altitude the captain decides with ATC.
The captain does not decide what level they leave the stack at. The approach controller manages the stacks, gives vectors from the stacks and descends the aircraft. Normally the aircraft will leave the stack at min stack level, FL70 or 80 normally (depending on the QNH)
The changes they wanted to make would have decreased the noise heard for many people, kept the aircraft higher for longer and decreased the area in which the aircraft were flying. It would have also given a separate stack for Luton inbounds and therefore keeping them higher for longer instead of being restricted to 5A flying E to W over Duxford/Fowlmere/Royston area which happens now.
How is separation from gliders maintained?
You don't, it non-controlled airspace